Electrically heated pit furnace,particularly for melting vitreous silica

ABSTRACT

To provide separately controllable temperature zones in the furnace, for melting of the silica, and for subsequent drawing into tubing or cane, heater wires, arranged in a ring around the crucible are formed to have two different cross sections, one being thick to form connecting portions and the other thin, to form heating portions, the thin cross sections of the heater wires being arranged adjacent the zones to be heated, the thick connecting portions passing through the other zones; the heater wires for the various zones are commonly connected at one axial end to a common connection ring, and at the other axial end to separate connection rings, for separate energization in accordance with heating requirements.

United States Patent Vatterodt et a1. I 1March 20, 1973 1 ELECTRIICALLY HEATED PIT 2,596,327 5/1952 Cox et al. ..2l9/381 FURNACE, PARTICULARLY F R 3,181,845 5/1965 Malm et al ..13/22 x MEL'HNG VITREOUS SILICA 1,646,599 10/1927 Schaefer ..219/300 UX Inventors: Karl Vatterodt; Werner Weiss, both of Berlin, Germany Patent-Treuhand-Gesellschaft Elektrische Gluhlampen Munich, Germany Filed: Dec. 1, 1971 Appl. No.: 203,574

fur mblll,

Assignee:

Foreign Application Priority Data Dec. 31), 1970 Germany ..P 20 64 433.]

References Cited UNITED STATES PATENTS Vatterodt 1 3/6 X Primary Examiner-Roy N. Envall, Jr. Att0rneyRobert D. Flynn et al.

[5 7 ABSTRACT To provide separately controllable temperature zones in the furnace, for melting of the silica, and for subsequent drawing into tubing or cane, heater wires, arranged in a ring around the crucible are formed to have two different cross sections, one being thick to form connecting portions and the other thin, to form heating portions, the thin cross sections of the heater wires being arranged adjacent the zones to be heated, the thick connecting portions passing through the other zones; the heater wires for the various zones are commonly connected at one axial end to a common connection ring, and at the other axial end to separate connection rings, for separate energization in accordance with heating requirements.

1 1 Claims, 2 Drawing Figures sum 1 BF 2 v K rl V it t a a erod INVENTORS Dl'. Werner WeiB mmmmzoxm SHEET 2 [IF 2 FIG. 2

Karl Vatferodt I Dr Werner. w ir;

lwm'roks ELECTRTCALLY HEATED PIT FURNACE, PARTllCULARLY FOR MELTING VITREOUS SILICA The invention relates to pit furnaces, for example for melting vitreous silica or quartz-resembling glasses of high silica content, and more particularly to a pit furnace having an electrically heated crucible.

The furnace of the present invention is particularly useful for the continuous production of quartz tubing, where the raw material is filled in at the top of a cylindrical crucible and the molten vitreous silica is shaped to tubing in a nozzle, the nozzle forming the bottom of the crucible. The tubing is discharged at the bottom. The quartz tubing leaving the furnace can be seized by a drawing machine located below the furnace and be drawn out to receive its final shape.

The heater filaments of the crucible are commonly of a refractory metal such as molybdenum or tungsten and are usually arranged to form a ring within furnace, furnace surrounding the crucible symmetrically with respect to its axis. During operation, a stream of protective gas such as hydrogen or a mixture of hydrogen and nitrogen is circulated around the heater filaments.

The continuous heater filaments are connected at the top and bottom ends to current supply rings and are jointly supplied with current. This type of heater filament arrangement permits a reduction or increase in temperature only throughout the entire region of the crucible. To draw the tubing, heating of the nozzle should be regulated independent of the crucible heating. In order to achieve such a heating of the nozzle independently of the crucible heating, the heater filaments were previously divided in the center so that an upper and a lower heater filament ring is obtained,

each ring being provided with an individual current supply which is independent of that of the other ring. Nevertheless it is difficult to design the heater filament arrangement so that no drop in temperature occurs at the transition point from the upper to the lower heater filament ring. Moreover, it is almost impossible to locate additional current supply conductors in the center, since they would melt due to the heat radiating from the furnace.

It is the object of the invention to provide a heating arrangement for the drawing nozzle which is independent of the crucible heating, in which the disadvantages described above are avoided and which provides a continuous temperature gradient along the level of the crucible.

Subject matter of the invention: A ring of heater filaments is provided. The heater filaments are rod-shaped and formed along their overall length such that they have connecting sections of thick diameter and heating sections of thin diameter. The transition point between connecting section and heating section may be in any place of the heater filaments, e.g. at half their length.

. The ring of heater filaments is arranged around the crucible such that the heater filaments for the heating of the upper furnace zone have thin upper heating sections and alternate with filaments which are axially reversed, that is, have the thin heating sections at their lower ends to heat the lower furnace zone independently of the other filaments. The connecting sections of the heater filaments should contribute only little to the heating of the respective furnace zone, and their diameter is to be designed accordingly. The diameter of the heater filament sections contributing to heating depends on the furnace output required, which may vary depending on its use.

Both heater filament systems can be regulated electrically independently of each other. Two electrically separated connecting rings are placed at the bottom end of the furnace. The heater filaments are tightly screw-fitted into the connecting rings. The upper one of these connecting rings has bores therein to pass the heater filament end portions for heating of the upper furnace zone therethrough, while insulated from the rings. The upper connection is common for both systems. The differential linear extension which occurs during start-up heating, is absorbed by two tension systems which are capable of individually tensioning the filaments and include spring pockets to compensate for the linear expansion as disclosed in U.S. Pat. No. 2,998,469.

The heater filament system of the present invention permits adjustment of the temperature of both zones substantially independently of each other, so that the melting temperature in the crucible and the temperature of the nozzle become adjustable independently of each other. This if of particular importance when dimensioning quartz tubing and maintaining proper tolerances. The set temperatures in both zones are affected only slightly by dissipation of Joule heat from the connecting sections, and the heat transfer due to the flushing gas.

The invention will be described by way of example with reference to the accompanying drawings, wherein:

FIG. 1 is a longitudinal sectional view of a pit furnace;

and FIG. 2 is a schematic circuit diagram of the furnace heating arrangement.

The electrically heated pit furnace is supplied at the top with the raw material which is melted to vitreous silica in the cylindrical crucible 1. The vitreous silica is shaped to tubing by a nozzle 3 in bottom 2; with another appropriate nozzle design, cane of round or any other cross section can be formed. The tubing discharged through the nozzle is seized and drawn out by a drawing machine until it is completely solidified. The crucible is surrounded by a ring of continuous heater filaments. To provide nozzle heating which is independent of the crucible heating, the heater filaments are shaped, preferably up to about half their length, i.e. at about the lever of the nozzle, such as to form heavy connecting sections 5, 5' of thick diameter; and thin heating sections 4, 4' of thin diameter, each forming a respective heating system for the upper and the lower crucible portions respectively. Filament 4-5 is heavy at the bottom 5 and thin at the top, to heat the top portion but provide practically no, or only little heat to the bottom, or nozzle portion of the furnace; while filament 4'5' has a heavy connecting section on top and is dimensioned to heat, with section 4, the nozzle zone or portion of the furnace. The number of heater filaments depends on the crucible diameter but in any case these must be even-numbered to provide an axially symmetrical arrangement of heater filaments with upper heating and lower connecting sections and vice versa (see FIG. 1). The dimensioning of the diameter of the heater filaments depends first of all on the furnace output required, but the connecting sections should be dimensioned such as to contribute as little as possible to the heating of the furnace zone.

The two heater filament systems for the heating of the upper and the lower crucible zone can be controlled independent of each other, and by this means the temperature of the upper crucible zone can be adjusted independent of the lower zone. This is important particularly for the drawing procedure, as the nozzle temperature is of decisive importance for the dimensioning of the quartz tubing. The temperature of the two zones is affected slightly by the dissipation of some Joule heat by the connecting sections, and also by the heat transfer by the flushing gas which surrounds the heater filaments.

The two heater filament systems are connected at the I bottom end of the furnace to two separate connecting Pat. No. 2,998,469).

EXAMPLE AND OPERATION A pit furnace constructed in accordance with FIG. 1 is rated for an electrical power requirement of 75 kva which is to be supplied to two heater filament systems,

each comprising 16 heater filaments of 300 mm each of active heating length. The heater filaments are made from tungsten and have a connecting section of 8 mm in diameter and a heating wire section of 2.8 mm in diameter. (The heater filaments of both types are actually somewhat longer due to the length of the end portions which are clamp or screw connected; the clamps do not materially affect the calculation'of electrical data and are thus not included in the effective lengths given above).

With a maximum total power of 75 kva, each heating system can be loaded with 37.5 kva at the most. ln-case of 10 V terminal voltage, a maximum current of 3,750 A results, and with the given heater filament dimensions, a surface area loading of 80 W/cm ensues. For supply, two transformer units 10, 10' (FIG. 2) of 60 kva each and of V secondary voltage are provided.

The excess capacity permits possible changes in the zone length or in the overall length of the furnace. The transformer units l0, l0 consist each of a variableratio transformer connected as auto-transformers, with about 10 rough and eight fine adjustment steps in combination with a phase reversing switch 11, 11 and a two-core heavy-current transformer 12, 12'. The heater sections of the filaments deliver approximately 90 percent of the total output and the connecting sec tions about 10 percent of the total output. This slightly additionally affects the temperature in both zones and leads to a blurring of the temperature maxima. The effect is of about the same magnitude as that produced by heat conduction and heat transfer.

Pit furnaces can be heated such as to havethree or even more independently regulable temperature zones. The heater filaments are then designed accordingly;

e.g. for three temperature zones, the heater filaments are of such a design that either the upper, the central or the lower third of'their overall length is designed as a heating section, whereas the rest of the heater filament is designed as a thick connecting section. The number of lower connecting rings as well as the number of rods forming the heater filament ring must be selected in accordance with the number of heating systems: with three temperature zones, three lower terminal rings are therefore required, and the total number of heaterfilaments must be divisible by three, in order to provide an axially symmetrical arrangement.

We claim: I

l. Electrically heated pin furnace for melting vitreous silica or quartz-resembling glasses of high silica content, comprising a crucible;

a circular drawing nozzle disposed in the bottom of the crucible for continuous extraction of tubing or cane;

a plurality of rod-like heater filaments arranged in a ring surrounding the crucible; I

means supplying current to the rod-like heater filaments;

and means compensating for linear expansion of the filaments;

wherein the rod-like heater filaments are formed along their axial length to have a connecting section of thick diameter and a heating section of a diameter thin with respect to that of the connecting section to provide essentially all heat energy upon passage of current in the zone of the thin heating section; wherein an even number of said rod-like heater filaments are provided, alternately disposed heater filaments having heating sections alternately in the upper and in the lower portion of the filaments to form two filament systems; and means are provided individually electrically connecting the filament systems.

2. Furnace in accordance with claim 1, wherein the transition point between connecting section and heating section is at about half the length of the heater filament.

3. Furnace in accordance with claim 1, wherein the diameter of the heater filament connecting section is so large as to contribute negligibly to the heating of the respective furnace zone.

4. Furnace in accordance with claim 1, wherein the heater filaments are made of tungsten.

5. Furnace in accordance with claim 1, wherein the means separately connecting the filament systems comprises two electrically separated connecting rings connected to the respective heater filaments and a common connecting ring located at the top of the furnace.

6. Furnace in accordance with claim 1, providing a melting zone of a first temperature and at least one more zone at a second temperature;

the filaments having the heating section in the upper portion forming a first heating system and being arranged to have the heating section thereof located axially adjacent one of the melting zones; and the filaments having the heating section in the lower portion forming another heating system being arranged to have the heating sections thereof located axially adjacent the other zone.

9. Furnace according to claim 8, including separately controllable current supply means connected to each said rings.

10. Furnace according to claim 6, wherein two systems are provided, and the filaments of the systems are axially reversed with respect to each other.

11. Furnace according to claim 1, wherein the filament sections are dimensioned to provide, upon energization, about of heat from the heating section. 

2. Furnace in accordance with claim 1, wherein the transition point between connecting section and heating section is at about half the length of the heater filament.
 3. Furnace in accordance with claim 1, wherein the diameter of the heater filament connecting section is so large as to contribute negligibly to the heating of the respective furnace zone.
 4. Furnace in accordance with claim 1, wherein the heater filaments are made of tungsten.
 5. Furnace in accordance with claim 1, wherein the means separately connecting the filament systems comprises two electrically separated connecting rings connected to the respective heater filaments and a common connecting ring located at the top of the furnace.
 6. Furnace in accordance with claim 1, providing a melting zone of a first temperature and at least one more zone at a second temperature; the filaments having the heating section in the upper portion forming a first heating system and being arranged to have the heating section thereof located axially adjacent one of the melting zones; and the filaments having the heating section in the lower portion forming another heating system being arranged to have the heating sections thereof located axially adjacent the other zone.
 7. Furnace in accordance with claim 6, including separate current supply means for at least the first and another system.
 8. Furnace in accordance with claim 6, wherein the means to supply current to the filaments includes axially spaced connection rings, one for each system and having the filaments thereof connected thereto, any ring having a ring therebelow being formed with openings through which the filaments, connected to the ring therebelow, pass in insulated manner.
 9. Furnace according to claim 8, including separately controllable current supply means connected to each said rings.
 10. Furnace according to claim 6, wherein two systems are provided, and the filaments of the systems are axially reversed with respect to each other.
 11. Furnace according to claim 1, wherein the filament sections are dimensioned to provide, upon energization, about 90% of heat from the heating section. 